This application is based upon and claims the benefit of priority of Japanese Patent Applications No. 2000-24919 filed on Feb. 2, 2000, No. 2000-25191 filed on Feb. 2, 2000, No. 2000-283360 filed on Sep. 19, 2000, and No. 2000-369722 filed on Dec. 5, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a motor, in particular, a motor having a worm and a worm wheel for speed reduction (speed reduction device), which is applicable to a power window or a sunroof.
2. Description of Related Art
Conventionally, a motor to be used in a power window system has a motor body and a speed reduction device. The speed reduction device has a gear housing in which a worm shaft coupled coaxially with a motor shaft of the motor body and a worm wheel in mesh with a worm formed in the worm shaft are housed. Opposite ends of the worm shaft are rotatably held by bearings fixed in the gear housing.
When the motor shaft is driven to rotate, the worm shaft rotates along with the rotation of the motor shaft so that worm wheel rotates at a lower speed and with a higher torque than the worm shaft. Accordingly, an output shaft connected to the worm wheel rotates to transmit its rotational force to an outside load. Such a motor is applicable not only to the power window system but also the other various systems in which the output shaft rotates at a low speed and with a high torque.
However, when an overload is applied to the output shaft during the rotation of the motor, the worm shaft receives a large bending force in a perpendicular direction thereto (in an opposite direction to a position where the worm wheel is located). Therefore, the bending force together with a rotating force transmitted from the motor shaft causes the worm shaft to bend. Accordingly, the conventional motor has a drawback that locally concentrated frictional wear is likely to occur in the bearings rotatably holding the worm shaft or the gear housing made of resin is likely to deform, resulting in reducing a motor efficiency and generating noises.
An object of the invention is to provide a motor in which local frictional wear of bearings rotatably holding the worm shaft and deformation of a gear housing are limited.
To achieve the above objects, a motor has a speed reduction devise composed of gear housing, a worm wheel, a worm shaft with a worm, and first and second bearings rotatably supporting the worm shaft on opposite sides of the worm. With the motor mentioned above, when the worm shaft is bent by an overload transmitted thereto via the worm wheel, an axial inner surface of the first bearing maintains an axially widespreading face contact with the worm shaft in a manner that the first bearing inclines a given angle toward a bending side of the worm shaft to follow the bending of the worm shaft or in a manner that the axial inner surface of the first bearing is formed in a shape of approximately following a bending shape of the worm shaft and actually contacts the worm shaft at positions more extending axially toward the worm.
It is preferable that the gear housing has a bearing holding portion protruding outwardly therefrom to an extent that the bearing holding portion readily inclines along with the first bearing fixed thereto in order to follow the bending of the worm shaft.
Further, preferably, the first bearing is provided with a first cylindrical inner circumferential portion whose inner diameter is axially constant and a first tapered inner circumferential portion axially adjacent to the first cylindrical inner circumferential portion on a side of the worm, an inner diameter of the first tapered inner circumferential portion is larger toward an opposite side to the first cylindrical inner circumferential portion. In the motor having the first bearing mentioned above, the axial inner surface of the first bearing actually in contact with the worm shaft is normally the first cylindrical inner circumferential portion and, when the worm shaft is bent by a given amount, extends up to at least a part of the first tapered inner circumferential portion.
It is preferable that the second bearing has also a second cylindrical inner circumferential portion and a second tapered inner circumferential portion whose constructions are same as those of the first bearing. The First and second tapered inner circumferential portion are arranged to face each other on opposite sides of the worm. Therefore, the respective axial inner surfaces of the first and second bearings are formed in a shape of approximately following the bending shape of the worm shaft and actually contact the worm shaft at positions more extending axially toward the worm, when the worm shaft is bent.